Acute leukemia arising in the setting of a preexisting myelodysplastic syndrome is especially resistant to treatment. The only curative treatment for these disorders is bone marrow transplantation, a treatment that remains toxic and available only to a subset of patients. Chronic myelomonocytic leukemia (CMML) is a myelodysplastic syndrome characterized by monocytosis, variable bone marrow fibrosis, and progression to acute leukemia. The t(5;12) is associated with CMML and results in the expression of a chimeric protein fusing the amino-terminus of TEL with the intracellular portion of the PDGFbetaR. Our lab cloned the TEL/PDGFbetaR fusion gene and has extensively characterized its in vitro properties. TEL/PDGFbetaR is constitutively tyrosine phosphorylated and forms oligomers mediated by the TEL pointed (PNT) domain in vitro and in COS cells. In Preliminary Results the candidate describes the development of two animal models of hematopoietic malignancy developed using TEL/PDGFbetaR. Mice transplanted with murine bone marrow (BMT) with TEL/PDGFbetaR introduced by retrovirally mediated gene transfer develop a rapidly fatal myeloproliferative syndrome. Furthermore, transgenic mice that have TEL/PDGFbetaR targeted to the lymphoid compartment by the immunoglobulin heavy chain promoter-enhancer (EmuVHP) develop lymphoblastic lymphomas. The focus of this proposal is to increase our understanding of the molecular mechanisms underlying transformation by TEL/PDGFbetaR in these murine models. The specific aims are: 1) To characterize the signaling pathways activated by TEL/PDGFbetaR in vivo, 2) To determine the relevance of activated signaling pathways to the development of hematopoietic malignancy in mice, and 3) To identify the downstream targets of TEL/PDGFbetaR relevant to the transformation of hematopoietic cells. A series of signaling mutants of TEL/PDGFbetaR will be used in repeat BMT experiments to determine residues critical for in vivo transformation. Using coimmunoprecipitation and gel shift assays, the activity of signaling molecules in the Ras and JAK/STAT pathways will be analyzed. Lastly, DNA microarray technology will be used to examine genomic expression of downstream targets of TEL/PDGFbetaR and its mutants in cultured cells. These experiments will provide a detailed understanding of the mechanisms of transformation by TEL/PDGFbetaR, and will provide a basis for the development of rational therapies to cure or halt the progression of CMML.